Abstract in another language

The spin dynamics in optically excited paramagnetic diluted magnetic semiconductors is investigated. To this end, a quantum kinetic density matrix theory which was developed by Christoph Thurn is applied, analyzed and extended.
Earlier studies which mainly concentrated on the case of a vanishing magnetization of the magnetic impurities revealed that the spin dynamics of optically excited electrons in three-dimensional systems is well reproduced by rate equations, where the rates can be derived from the Markovian limit of the quantum kinetic equations and coincide with the result of Fermi’s golden rule. In two-dimensional systems, however, deviations between quantum kinetic simulations and results of Markovian rate equations in the form of non-monotonic overshoots of the carrier spin polarization below its asymptotic value for long times have been discovered.
In the present thesis, first, Thurn’s quantum kinetic theory is applied to the case of finite impurity magnetization and equations in the Markovian limit are derived which reproduce well the quantum kinetic results and whose form has notable similarities to Landau-Lifshitz-Gilbert equations. The derived effective equations are then applied to study the competition between the spin-orbit coupling and the carrier-impurity exchange interaction. For this purpose, the quantum kinetic equations are extended and in addition to the exchange interaction, also k-dependent effective fields together with carrier and impurity Zeeman energies are accounted for. This further enables the derivation of
explicit expressions for the magnetic-field dependence of the spin transfer rates from the quantum kinetic equations. In contrast to the prevalent theories in the literature, the rate equations obtained here conserve the single-particle energies.
The causes and conditions for the appearance of non-Markovian effects are investigated more thoroughly. It is found that the non-Markovian behavior of the spin dynamics is particularly pronounced if carriers are excited in close proximity to the band edge. Accounting explicitly for the correlations between carriers and impurities in the quantum kinetic theory enables a discussion of genuine many-body effects like a renormalization of the precession frequency of the carrier spins for a finite impurity magnetization and a build-up of correlation energy.
Subsequently, the optical excitation of diluted magnetic semiconductors is taken into account on a quantum kinetic level in order to identify optimal excitation conditions for the detection of non-Markovian effects. Furthermore, it is investigated whether an efficient control of the spin dynamics in semiconductors with spin-orbit interaction by excitation with light with orbital angular momentum (twisted light) is possible. However, it is found that in extended systems, the spin dynamics after the optical excitation is nearly independent of the orbital angular momentum of the light.
Finally, the quantum kinetic theory is extended to account also for the scattering of carriers at a non-magnetic impurity potential which, in addition to the magnetic carrier-impurity interaction, originates from the doping with magnetic ions. It is found that the non-magnetic scattering leads to a redistribution of carriers in k-space, which can strongly isuppress some of the non-Markovian effects. Simultaneously, the build-up of strong non-magnetic correlations also results in a considerable enhancement of genuine many-body effects and increases the regime of parameters in which a significant renormalization of the carrier spin precession frequency can be expected.